The present invention relates to a package for SMT (Surface Mount Technology) type SAW (surface acoustic wave) filter, and more particularly to a conductor pattern structure of a substrate having a sufficient electrical isolation between input and output in a high frequency band of at least 400 MHz.
This kind of SAW filter is capable of providing a sharp cut-off characteristic for a desired pass band characteristic and has the possibility of light weight fabrication with a thin flatness form. Therefore, it can be preferably used as an IF filter for a receiver for satellite TV broadcasting or a filter for a moving body communication device such as a mobile telephone, a hand-held cellular telephone or a pager. Each of the parts of those devices has a strong demand for light weight, small thickness, short length and small size. The SAW filter is an element fabricated using a piezoelectric substrate which may include 36.degree. rotated Y cut X propagation LiTaO.sub.3 as a substrate, and some improvements thereof have been disclosed by JP-A-56-132807, JP-A-57-202114, JP-A-59-58907, etc.
The conventional package for SMT type SAW filter is provided with a plurality of patterns for terminals on an insulating substrate, as has been disclosed by JP-A-61-245709, but is not provided with a structure for preventing or blocking an electromagnetic interference between the patterns. That is, it is apparent that isolation for preventing electromagnetic interference is not a concern in this example noting that a shield for preventing electromagnetic coupling is not provided, such as between the input and output terminals. Also, a structure such as that shown in FIG. 11A is known, that is a structure in which at least one ground terminal E1 on the input side and at least one ground terminal E2 on the output side are provided in addition to a SAW filter chip SF, an input terminal IP and an output terminal OP which are provided on the upper surface (or front face) of a substrate P made of a ceramic material. However, the ground terminals E1 and E2 are not terminals for preventing an electromagnetic coupling between the input and output terminals IP and OP. Namely, the ground terminals E1 and E2 are not provided with a sufficient coupling prevention function since the input and output terminals IP and OP extend from the upper surface of the substrate P to the lower surface thereof and are disposed opposite to each other even on the lower surface of the substrate, as is best shown in FIG. 11B which shows the lower surface side (or back face) of the substrate P. Those portions of the terminals which exist on the lower surface of the substrate are only to be connected with peripheral circuits. For a low frequency band not higher than 70 MHz, there is no problem for the structure shown in FIGS. 11A and 11B. However, for a high frequency band of at least 400 MHz, a sufficient electrical isolation between input and output cannot be obtained and hence there arises a problem that the out-of-band rejection deteriorates due to electromagnetic coupling between the input and output terminals.